In conventional firearms of this type, the slidable breechblock or carriage is held in place by a retaining member which is hinged to the gunstock and is normally latached thereto. With carriage encased in a housing, a manually operable arming button with a stem traversing the retaining member must be provided to facilitate the loading of the first cartridge from a magazine clip into the breech behind the gun barrel; this button must be removed prior to disassembly for inspection and cleaning purposes. Even where the carriage is not encased, such disassembly is rather cumbersome in the known automatic pistols of this type and generally requires removal of the barrel from the gunstock. Finally, the need for a rear carriage stop on the gunstock prevents a mounting of the hammer at an external, readily accessible location behind the breechblock.